A wavelength division multiplexed-passive optical network (WDM-PON) provides a high speed broadband communication service by using an inherent wavelength assigned to each subscriber. Accordingly, each subscriber receives a signal having a different wavelength corresponding thereto, so that a security is enhanced and a separate communication service is provided to each subscriber, thereby enlarging a communication capacity.
Conventionally, a method has been proposed wherein a central office and a subscriber terminal have a respective light source including a distributed feedback-laser diode (DFB-LD) element, thereby realizing the WDM-PON.
However, such method has problems that the DFB-LD element is expensive and a temperature control technique is complicated.
Accordingly, a technique using a wavelength-locked optical signal has been widely used by injecting an incoherent light source into a Fabry-Perot Laser Diode (FP-LD) of a low price, thereby implementing an injection-locked WDM optical signal.
Hereinafter, a configuration of a conventional wavelength division multiplexed-passive optical network 100 will be described in reference to FIG. 1. FIG. 1 shows a schematic block diagram for showing a conventional bidirectional communication in an injection-locked wavelength division multiplexed-passive optical network.
The injection-locked wavelength division multiplexed-passive optical network 100 includes a central office 110, a subscriber terminal 130, a remote node 120 for connecting the central office 110 with each subscriber terminal 130 and an optical cable 140.
The central office 110 has an A band injection light source 111, a B band injection light source 112, a light source distributor 113, a first 1×N optical multiplexer/demultiplexer 114 and a multiplicity of transceivers 115.
The remote node 120 has a second 1×N optical multiplexer/demultiplexer 121 and the subscriber terminal 130 has a plurality of transceivers 131.
The A band injection light source 111 is provided as a light source for an A band optical signal serving as a downstream optical signal. As the A band injection light source 111, an incoherent light source may be mainly used. The A band injection light source 111 generates the A band injection optical signal, and then transmits it to the light source distributor 113.
The B band injection light source 112 is provided as a light source for B band optical signal serving as an upstream optical signal, and, like the A band injection light source 111, an incoherent light source may be mainly used as the B band injection light source 112. The B band injection light source 112 generates the B band injection optical signal, and then transmits it to the light source distributor 113.
The light source distributor 113 receives the A band injection optical signal from the A band injection light source 111 and transmits it to the first 1×N optical multiplexer/demultiplexer 114 of the central office 110. Further, the light source distributor 113 receives a wavelength-locked A band optical signal from the first 1×N optical multiplexer/demultiplexer 114 of the central office 110 and transmits it to the optical cable 140 connected to the remote node 120.
In addition, the light source distributor 113 receives the B band injection optical signal from the B band injection light source 112 and transmits it to the second 1×N optical multiplexer/demultiplexer 121 of the remote node 120 through the optical cable 140. Further, the light source distributor 113 receives a wavelength-locked B band optical signal from the second 1×N optical multiplexer/demultiplexer 121 of the remote node 120 and transmits it to the first 1×N optical multiplexer/demultiplexer 114 of the central office 110.
The first 1×N optical multiplexer/demultiplexer 114 separates the A band optical signal received from the light source distributor 113 according to the wavelength thereof, and then, injects it to each transmitter of the transceivers 115 of the central office 110. For example, as the first 1×N optical multiplexer/demultiplexer 114, an arrayed waveguide grating (AWG) may be used.
As the transmitter of the transceivers 115, the Fabry-Perot Laser Diode (FP-LD) may be used and the transmitter generates the downstream optical signal to be transmitted to each subscriber.
Specifically, if the A band injection optical signal separated based on the wavelength thereof is injected to each transmitter of the transceivers 115, wavelength elements having a wavelength different from that of the injected optical signal are suppressed and wavelength elements having a wavelength equal to that of the injected optical signal is locked, thereby outputting the wavelength-locked A band downstream optical signal.
Each receiver of the transceivers 115 receives a wavelength-locked B band upstream optical signal from the subscriber terminal 130, and then, converts it into an electrical signal. A photo diode (PD) may be used as the receiver of the transceivers 115.
The second 1×N optical multiplexer/demultiplexer 121 of the remote node 120 separates the B band optical signal received from the light source distributor 113 based on the wavelength thereof, and then, injects it to the transceivers 131 of the subscriber terminal 130. The arrayed waveguide grating (AWG) may be used as the second 1×N optical multiplexer/demultiplexer 121 like the first 1×N optical multiplexer/demultiplexer 114.
The Fabry-Perot Laser Diode (FP-LD) may be used as the transmitter of the transceivers 131, for example, and the transmitter generates an upstream optical signal to be transmitted to the central office 110.
Specifically, if the B band injection optical signal separated according to the wavelength thereof is injected to the transmitter of the transceivers 131, wavelength elements having a wavelength different from that of the injected optical signal are suppressed and wavelength elements having a wavelength equal to that of the injected optical signal is locked, thereby outputting the wavelength-locked B band upstream optical signal.
Each receiver of the transceivers 131 receives the wavelength-locked A band downstream optical signal from the central office 110, and then, converts it into an electrical signal. A photo diode (PD) may be used as the receiver of the transceivers 131.
As described above, the transmitter for use in the wavelength division multiplexed-passive optical network (WDM-PON) outputs an optical signal including data.
FIG. 2 depicts characteristics of a laser diode LD used in an optical transmitter.
The laser diode LD 200 generates an optical signal having an optical power P 202 according to a current I 201 applied from a driving circuit (not shown). The laser diode LD 200 is, for example, a diode that generates an optical signal by using a forward semiconductor junction as an active medium, and a material thereof may be GaAs or the like.
The optical power P 202 of the optical signal generated by the laser diode LD 200 has a relationship shown as a graph 200a in FIG. 2 with respect to the current I 201 applied to the laser diode LD 200. That is, the optical power P 202 has a value of 0 in a case that the current I 201 is under the value of a threshold current. However, when the current I 201 exceeds the value of the threshold current, the optical power P 202 increases in proportion to the current I 201.
The laser diode LD 200 receives a driving current 206 and generates an optical signal having an output optical power 207 according to the relationship between the current I 201 and the optical power P 202 shown in the graph 200a. 
Therefore, when the temperature is T1, the laser diode LD 200 generates an optical signal having an output optical power of P0 if it receives a current I0, and generates an optical signal having an output optical power of P1 if it receives a current I1.
In other words, when the temperature is T1, a bias current I_bias1 204 of the laser diode is I0, and a modulation current I_mod1 205 thereof is I1-I0.
However, the laser diode LD 200 generates optical signals having different optical powers with respect to an identical driving current depending on the surrounding environment, such as the variation of the temperature or the deterioration of the laser diode LD.
For example, when the temperature is T1 as described above, the laser diode LD 200 generates optical signals having the optical powers P0 and P1 in case of receiving I0 and I1, respectively. However, when the temperature is T2, I2 and I3 need to be applied to the laser LD 200 instead of I0 and I1 as a driving current so as to generate optical signals having the output optical powers P0 and P1, respectively.
That is, when the temperature is T2, a bias current I_bias2 208 and the modulation current I_mod2 209 of the laser diode LD 200 are I2 and I3-I2, respectively.
Thus, the laser diode LD 200 should be applied with driving currents having different values depending on the surrounding environment such as temperature in order to generate identical output optical powers.
Accordingly, an optical transmitter that detects an optical signal outputted from the back facet of the laser diode and calculates the value of the driving current to be applied thereto according to the detected optical signal has been conventionally used. FIG. 3 shows a configuration of a conventional optical transmitter, a relationship between a driving current and an optical power and a relationship between the driving current and a monitoring current.
The conventional optical transmitter that controls the driving current includes a laser diode LD 200, a photo diode PD 220 and a driving circuit 230.
The back facet of the laser diode LD 200 is coated with a material that has a high reflectivity. Thus, most of the optical signal generated by the laser diode LD 200 is outputted through a front facet thereof, while a very small amount of the optical signal is outputted to the back facet thereof. At this time, the optical power of the optical signal outputted from the back facet of the laser diode LD 200 is proportional to that outputted from the front facet thereof.
The photo diode PD 220 receives the optical signal outputted from the back facet of the laser diode LD 200 and then converts the received optical signal into a monitoring current Im 203. The converted monitoring current Im 203 is proportional to the optical power of the optical signal received by the photo diode PD 220, and the photo diode PD 220 transmits the monitoring current Im to the driving circuit 230.
The driving circuit 230 receives the monitoring current Im 203 from the photo diode PD 230, and calculates the optical power of the optical signal outputted from the front facet of the laser diode LD 200 based on the received monitoring current Im 203. Then, the driving circuit 230 controls the value of the driving current I 201 that needs to be applied to the laser diode LD 200 based on the calculated optical power. The laser diode LD 200 generates an optical signal according to the controlled driving current I 201.
The relationship between the driving current I 201 and the optical power P 202 is shown in the graph 200a, and the relationship between the driving current I 201 and the monitoring current Im 203 is shown in the graph 220a. As shown in the graphs 200a and 220a, the optical power of the optical signal actually outputted from the front facet of the laser diode LD 200 is in proportion with the monitoring current Im 203 outputted from the photo diode PD 220.
In the meantime, it is required that an average power and an extinction ratio ER(=P1/P0) of an optical signal received by an optical receiver are controlled to fall within a predetermined range in order to accomplish an optimal performance in the wavelength division multiplexed-passive optical network (WDM-PON) as well as in a general optical transmission system. Accordingly, the driving circuit 230 performs automatic power control (APC) and automatic ER control (AEC) based on the monitoring current Im, so that a bias current and a modulation current are adjusted until the average power and the extinction ratio ER fall within a desired range.
FIG. 4 depicts a WDM-PON implemented by using a conventional optical transmitter in which the driving current is controlled according to the prior art.
The optical transmitter 310 includes a laser diode 200 and a monitoring photo diode mPD 220. The optical transmitter 310 receives an injection light from a 1×N optical multiplexer/demultiplexer 320, generates a wavelength-locked optical signal by using the received injection light, and transmits it back to the 1×N optical multiplexer/demultiplexer 320.
The laser diode LD 200 generates an optical signal according to a driving current applied by a driving circuit (not shown) to transmit most of the optical signal to the 1×N optical multiplexer/demultiplexer 320 through the front facet thereof and a very small amount of the optical signal to the monitoring photo diode mPD 220 through the back facet thereof.
The monitoring photo diode mPD 220 receives the optical signal outputted from the back facet of the laser diode LD 200; converts the optical signal into the monitoring current Im; and then transmits the monitoring current Im to the driving circuit (not shown). The driving circuit calculates the value of the driving current that needs to be applied to the laser diode LD 200 based on the monitoring current Im.
The 1×N optical multiplexer/demultiplexer 320 divides the injection light received from a light source distributor 330 according to the wavelength thereof and then injects the divided injection light to the optical transmitter 310. Moreover, the 1×N optical multiplexer/demultiplexer 320 performs band-pass filtering on each of the wavelength-divided optical signals received from the transmitter 310 to transmit it to the light source distributor 330.
Thus, the driving current to be applied to the laser diode LD 200 has been controlled based on the optical signal outputted from the back facet of the laser diode LD 200 in accordance with the conventional WDM-PON system.
However, the optical signal received by the monitoring photo diode mPD 220 has wavelength bands corresponding to each subscriber and the residual wavelength bands not corresponding to each subscriber. In contrast to this, the optical signal that is received by the optical receiver located in the subscribers side only has the wavelength bands corresponding to each subscriber as depicted in a spectrum 331 since the optical signal is filtered according to the wavelengths thereof by the 1×N optical multiplexer/demultiplexer 320.
Therefore, in accordance with the conventional driving current control system, there have been difficulties in performing precise control of the driving current since the spectrum 331 of the optical signal received by the receiver (not shown) in the subscribers side is not identical to a spectrum 221 of the optical signal serving as a basis for the control of the driving current. Moreover, this discrepancy between the spectrums 221 and 331 occurs very differently depending on the temperature, the intensity of the injection light source and the characteristics of the laser diode LD. Thus, this causes a deterioration of a system performance in the WDM-PON since the implementation of the APC function and the AEC function available in the other optical transmission system cannot be completely achieved.